Light-emitting diodes (LEDs) have already been used for quite some time as light sources in luminaires. Light-emitting diodes are distinguished by low power consumption and a long service life. Meanwhile, it has also become known to use light-emitting diodes in street lightings. For this purpose, individual light-emitting diodes or groups of light-emitting diodes can be used. For influencing the emission characteristics of the light-emitting diodes, the latter are normally equipped with light-directing means of a transparent nature. Collimators, ancillary lenses, diffusion disks or the like may, for example, be used for this purpose. The light-directing means have the effect that the light generated in the light-emitting diode is concentrated in one spatial direction. In addition, the light beam has imparted thereto a specific distribution. Examples for such distributions are e.g. a concentrating distribution, a scattering distribution or a banded distribution. Due to the light-directing ancillary unit, each light-emitting diode or each group of light-emitting diodes becomes a very small spotlight with special light-technical characteristics. The emission directions of the individual light-emitting diodes or of the groups of light-emitting diodes are determined by tilting the light-emitting diode or the group of light-emitting diodes and by their positioning in the luminaire housing of the street lighting. The light-emitting diodes or groups of light-emitting diodes are directly oriented onto the target surface, e.g. onto the surface of a carriageway. Individual light-emitting diodes or groups of light-emitting diodes irradiate light onto different points of the target surface. Due to the superimposition of the individual light emissions of the light-emitting diodes or of the groups of light-emitting diodes, the desired luminosity distribution is achieved on the target surface.
This arrangement of light-emitting diodes in a street lighting is, on the one hand, disadvantageous insofar as the light-emitting diodes illuminate the target surface directly and are therefore also directly visible. The very small dimensions and high luminous fluxes of the individual light-emitting diodes lead to very high luminous densities on the surfaces of the light-emitting diodes or on the ancillary optics of the light-emitting diodes. This leads to a strong glare for an observer.
Since the light-emitting diodes or the groups of light-emitting diodes are individually oriented onto points of the target surface, a very complex geometry of the mechanical structure of the luminaire is required. In addition, the light-emitting diodes must be wired and mounted individually or in several groups. This results in a high manufacturing outlay and therefore also in high costs of the overall system. Hence, also the repair of the light-emitting diode unit entails great effort and high costs.
Another drawback is to be seen in the collimators which are frequently used for concentrating the light of the light-emitting diodes. The collimators have a comparatively low efficiency amounting in some cases only to approx. 75%. Street lightings having the above-described structural design are therefore often inefficient. Another drawback is to be seen in the fact that most collimators operate on the basis of the principle that the light emitted by the light-emitting diode is totally reflected by the circumferential surface of the collimators. If water droplets or condensed moisture adheres to the circumferential of surface the collimators, the collimators will be rendered ineffective. Hence, street lightings in which the ancillary optics of the light-emitting diodes consist of collimators tend to be failure prone in the case of an ingress of moisture.
In addition, it is also known to use so-called light-emitting diode clusters (LED clusters) as a light source in street lightings. LED clusters consist of individual light-emitting diodes which are combined so as to form a homogeneous group of light-emitting diodes. The light-emitting diodes are often arranged in common on a conductor board. Most of the ray beams of the individual light-emitting diodes therefore have the same direction, so that the LED cluster can be seen as a single light source and can therefore also be compared with a conventional light source. The light emitted by the whole LED cluster is then conducted through ancillary optics. For example, the cover glass of the street lighting may be configured as ancillary optics. It is possible to produce the cover glass from moulded glass having incorporated therein light-directing structures, e.g. lens-shaped or prismatic elements.
A street lighting of this kind is, on the one hand, disadvantageous insofar as it will normally not be possible to generate the ideal-typical luminosity distributions of a street lighting. The distributions generated are, however, those known from the field of headlight construction and automotive engineering. If the light-directing structure is configured as a prismatic structure, a banded distribution will normally be accomplished. This variant is not very desirable for use as street lighting. In addition, the efficiency of such a system must be considered rather low.